Conventionally, portable lighting devices such as flash lights use incandescent lamps as light sources. In recent years, light emitting diodes (LEDs) has become popular in LCD backlight, home appliance, and street light applications. The adoption of the LEDs for flash lights has been increased due to LEDs' better light efficiency and longer life over incandescent lamps.
Flash lights are usually powered by batteries. The surge power applied to the lamps when the flash light is initially turned on may degrade the life time of the lamps. One of the common solutions is to add a current limiting resistor between the lamp and the battery. However, the power dissipation of the resistor may shorten the battery life.
The LED generally has a forward voltage between 3.2V to 4.0V when conducted. An alkaline battery cell for home appliances normally provides a voltage of 1.5V. Therefore, it may require at least three alkaline battery cells to power an LED. FIG. 1A shows a circuit 100 used in a conventional flash light. The circuit 100 uses a battery pack 110 including three series-connected cells as a power source. Each cell provides a voltage of 1.5V. The battery pack 110 powers an LED 130 via a switch 120. The LED 130 has a 3.2V forward voltage and a 100 mA current when conducted. The circuit 100 includes a current limiting resistor 140 (e.g., 13 Ohm) coupled between the LED 130 and the battery pack 110.
In operation, the power dissipation of the current limiting resistor 140 is approximately 0.13 Watt and the power dissipation of the LED 130 is approximately 0.32 Watt. As such, the power consumed by the LED 130 is approximately 71% of the total power provided by the battery pack 110. In other words, part of the battery power is wasted by the current limiting resistor 140. Thus, the battery pack 110 may need to provide sufficient power to maintain brightness of the LED 130, which may reduce the battery life.
Due to manufacturing process or other factors, the LED 130 may have a forward voltage of 4.0V when conducted. Thus, the current flowing through the LED 130 may be limited to approximately 38.5 mA, which is approximately 38.5% of the rated current (100 mA). Accordingly, the brightness of the LED 130 may be reduced to 38.5% of the expected brightness. The resistance of the resistor 140 can be changed from 13 Ohm to 5 Ohm to yield a current of 100 mA flowing through the LED 130 such that the LED 130 can have the expected brightness (the brightness when the LED current is 100 mA). However, if the resistance of the resistor 140 is 5 Ohm, the circuit 100 may overdrive the LEDs which have lower forward voltages. For example, for an LED having a forward voltage of 3.2V, the current flowing through the LED is approximately 260 mA which can be greater than a rated current of the LED. Consequently, the LED life time may be shortened.
FIG. 1B shows a graph 200 illustrating performance of the conventional circuit shown in FIG. 1A. The conventional circuit utilizes two 1.5V alkaline battery cells together with a current limiting resistor to drive an LED having a 100 mA rated current. As shown in the graph 200, the run time of the battery cells in this conventional circuit is only approximately 100 minutes.
Furthermore, the conventional circuit 100 is limited in practical applications when a user uses different LEDs with different power ratings. For example, the user may replace the LED having a 100 mA rated current with an LED having a 1A rated current with the expectation of obtaining greater power. Unfortunately, since the current limiting resistor has fixed resistance, the current flowing through the LED will not be changed. Moreover, the number of battery cells is usually determined by the shape of the flash light and can not be changed after production. Generally speaking, such conventional circuit using a current limiting resistor has lower power efficiency, lacks flexibility, and may not be practical for different applications.